how do I do a proper fuel pressure check?

I've been having an issue recently of the jeep not wanted to start immediately. Usually I'll crank it for a bit, stop, wait, start cranking again, and it will try to run, but will cut off, so I have to put my foot on the gas peddle and leave it on the peddle for a bit til' it can idle on it's own. If it does idle on it's own, it will be rough for a short amount of time, and then it will be fine. Drives fine and once warm it will start up first try.

I recently did an exhaust manifold change, had an idle issue which was an air leak, replaced the manifold gasket, fixed the idle issue. This no-starting issue creeped up a few days after my idle seemed to be fixed.

Last fri the CEL came on, the jeep threw these codes P0306 (cylinder 6 misfire) and P1294 (air/fuel metering - dunno the rest, but air/fuel mixture is off). I've tightened up the intake/exhaust manifold bolts, and the problem persists. I also reset the PCM just to make sure and the engine still runs odd. So I've narrowed it down to something fuel related, I bought a fuel pressure test guage and hooked it up to measure fuel pressure but I think I did it wrong b/c my readings were just.... not at all close to what I should be getting. I hooked the black hose end to the test port/schrader valve (per test guage directions) and the 6ft. clear plastic hose went into an empty container (for whenever I pressed the fuel release button). I have a feeling for this system that's a wrong way of testing....

Sorry for the extremely long list of events but I thought it good to state, anyway:

How do I properly check the fuel pressure? The haynes manual diagram was slightly confusing.... I'd like to narrow this down further.... since the fuel pump still delivers fuel I'm quite sure (75%) that it's something right at the fuel rail (possibly even dealing w/ injector #6 ). Please help me out, I've been chasing this for the past 2+ weeks.

Thanks in advance for all advice!

What year is it?

P1294 is target idle not reached.

Has it been tuned up recently, plugs, wires, etc? I would suspect plug wire and or fuel injector #6 as possibly having a problem.

How did you fix the idle issue?

If it runs fine once started and warmed up, it is not a fuel pump or fuel pressure problem. You test fuel pressure with a fuel pressure gauge (a dead ended dial gauge, no flow). They run $30 - 50.

It sounds like the IAC, Idle Air Controller motor/valve is sticking. Search this forum for the words "Idle Air Controller" and learn all about it. It is on the front left side of the throttle body.

sorry about lack of info issue:

98 4.0 AW4 - 142K

When pullin the codes at autozone, we were looking at the scrolling screen while I was writing down the details of the code, so if the target idle is still being reached, then... that sucks. I did plugs, wires, cap, and rotor around 136K.

I thought I had fixed the idle issue b/c it wasn't idling high as it had been previously whether in park or at a stop in drive. At startup it has been idling closing to 800-900 rpm which is the ideal idle. Before it was around 1500.

Is their a way to test the IAC? I also cleaned the throttle body (back during the high idle stuff) and I tried to take the IAC off but the torx bolt started to strip w/ just one turn.

bumpity bump.

From doin' a lil' searching I've gathered that some bosch spark plugs seem to suck.... I haven't changed mine yet (cuz I still view them as new) but don't mind doing so if that'll help. What kind of champion plugs are people running?

Testing fuel pressure is very simple. Screw on the fitting, look at the gauge. Note reading, disconnect vacuum line from regulator, reading should go up 5-10 psi. Reattach, shut off engine. Should still have about the same pressure after 5 minutes.

JJacobs said:

Testing fuel pressure is very simple. Screw on the fitting, look at the gauge. Note reading, disconnect vacuum line from regulator, reading should go up 5-10 psi. Reattach, shut off engine. Should still have about the same pressure after 5 minutes.

Click to expand...

disconnect vacuum line from regulator? If you could give me the location of these (I have a 98, now as many hoses and stuff as the 95 and below) and I'll gladly do some further research.

98 might not have one, not sure if it's a returnless system. Trust the gauge, if it doesn't read right something's messed up.

I'd love to do the guage test... but I can't tell if it's reading right if I can't test it correctly.

About it being a returnless system... I think so, but I've come across something about the BMW check valve. The symptoms I'm having seem to relate completely to the check valve failing and killing the fuel pressure inside the line:

Go to post #5:
http://www.jeepforum.com/forum/showthread.php?t=275249&highlight=check+valve

I changed out my spark plugs just to rule them out and I'm still having an issue. I would still like to know how to properly test out the fuel pressure w/o disconnecting the fuel line (if their is a way)


Thanks for all the help that has been offered!

98 will be a returnless system , pressure regulator is in the tank and is mechanical ( no vacuum line to it ) . also no test for the regulator .
running fuel psi should be 49 +/ - 5 psi .

sw_mi_xj said:

98 will be a returnless system , pressure regulator is in the tank and is mechanical ( no vacuum line to it ) . also no test for the regulator .
running fuel psi should be 49 +/ - 5 psi .

Click to expand...

ohhh.... I thought it was supposed to be 39... well shoot, if that's the case then I think I hooked it up correctly and my running pressure is fine.

Thanks for that! Now I just need to learn more about the bmw check valve thing and where to install it....

That's what I was getting it, it's really hard to screw up a fuel pressure test. Hook it up, make sure it's not leaking- look at the gauge. Simple.

jbassxj said:

ohhh.... I thought it was supposed to be 39... well shoot, if that's the case then I think I hooked it up correctly and my running pressure is fine.

Thanks for that! Now I just need to learn more about the bmw check valve thing and where to install it....

Click to expand...

On a 98 there would be no check valve since it is a returnless system, except maybe in the pump itself, right?

What pressure readings did you get. Did you measure it at idle and at say 2500 rpm? Both should about 49 psi.

The needle was closer to the 50 mark at idle. I'll have to take another test and rev it up. One of the only reasons I was asking was when I hooked it up w/ the engine off (warm from driving) the reading I was getting was under 10 psi, which I thought that was supposed to be closer to 31... So I got confused since I thought my idle was supposed to be closer to 39. I'll do some more checking and come back here.

edit:

Ecomike said:

On a 98 there would be no check valve since it is a returnless system, except maybe in the pump itself, right?

What pressure readings did you get. Did you measure it at idle and at say 2500 rpm? Both should about 49 psi.

Click to expand...

yeah, the 97+ are returnless, the check valve is inside the fuel pump sending unit which is beautifully placed inside the gas tank. So this weekend I'm gonna have to try and get the bmw check valve and hook that up, see if it does something nice.